Illumination homogenizing optical element

ABSTRACT

An illumination homogenizing optical element is disclosed that includes at least one optical surface having a plurality of refractive structures formed as either grooves or protrusions that, individually, are wider than the wavelength of light incident onto said illumination homogenizing optical element. At least part of the light incident onto the illumination homogenizing optical element is removed from the optical path by refraction of said light so as to provide even illumination on an illuminated surface. The illumination homogenizing optical element is advantageous over prior art illumination homogenizing optical elements in that it is easy and inexpensive to manufacture while providing sufficient optical performance.

This application claims the benefit under 35 U.S.C. 119 of JP2006-212,763 filed Aug. 4, 2006, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

In recent years, there have been increasing opportunities to photographsamples using a digital camera. Digital cameras using a sensor such as aCCD and a CMOS are more sensitive to brightness changes than are directobservations with human eyes or photographing with silver salt filmcameras. Because of this, non-uniformity in illumination, which is not aproblem in direct eye observations or photographing with silver saltfilm cameras, is a significant problem when photographing with digitalcameras. Therefore, in illumination devices for microscopes, there hasarisen a need to further increase the uniformity of illumination.

A conventional measure against non-uniformity in illumination is usingthe so-called Keller illumination technique, which theoreticallyprovides a uniform illumination on a sample surface. However, becausethe light intensity distribution over the angular direction of lightemitted by the light source is not uniform, non-uniformity inillumination still occurs. Although uniform illumination without anynon-uniformity in illumination can be obtained if the angulardistribution of light emitted from the light source can be made uniform,this is difficult to achieve in actuality.

As a means to improve the illumination non-uniformity caused by theangular distribution of light, a conventional method has been usedwherein an integrator, such as a fly-eye lens, is used to divide a lightflux into many parts; thus each part has a more uniform illumination.However, there has been the difficulty that adopting this method makesthe illumination optical system device itself large, thereby increasingthe cost.

Japanese Patent Application 2005-215992 proposes a method of correctinga non-uniform illumination distribution using an optical element such asa neutral density (hereinafter ND) filter and a frosted filter. Asimilar optical element is also disclosed in Japanese Laid Open PatentApplication 2006-30535. Each of these prior art examples of correcting anon-uniform illumination distribution has problems. For example, if anND filter is used, the influence of wavelength characteristics cannot beavoided. In other words, using an ND filter does not satisfy theobjective of reproducing accurate color by illumination with whitelight. On the other hand, if a frosted filter is used, there is theproblem such that it cannot be processed to have the exact transmittanceas calculated (through a numerical simulation). Namely, there remainsthe inaccuracy that the transmittance calculated in the design stagecannot be realized.

Moreover, in Japanese Laid Open Patent Application 2006-30535, anillumination distribution is corrected using an optical element having atransmittance distribution that directly complements (i.e., is theinverse of) the intensity distribution of the light source. Thus, thismethod of correcting a non-uniform illumination distribution does nottruly utilize the characteristics of the illumination optical system.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an optical element that correctsnon-uniformity in illumination light emitted from a light source, and adevice using the optical element. In order to solve the above discussedproblems, an optical element is provided which, in principle, has adifferent transmittance distribution from the optical element disclosedin Japanese Patent Application 2005-215992. Moreover, the opticalelement according to the present invention corrects a non-uniformdistribution of illumination while utilizing the optical characteristicsof the illumination system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below and the accompanying drawings, whichare given by way of illustration only and thus are not limitative of thepresent invention, wherein:

FIG. 1 shows a conventional illumination device;

FIG. 2 shows a filter inserted within a light path of the illuminationdevice shown in FIG. 1, for correcting for non-uniformity inillumination of the light source shown in FIG. 1;

FIG. 3( a) shows an example of a conventional illumination homogenizingneutral density filter, as viewed along the optical axis of the filter;

FIG. 3( b) illustrates the transmittance versus the distance from theoptical axis of the illumination homogenizing neutral density filtershown in FIG. 3( a);

FIG. 4( a) shows another example of a conventional illuminationhomogenizing neutral density filter, as viewed along the optical axis ofthe filter;

FIG. 4( b) illustrates the transmittance versus the distance from theoptical axis of the illumination homogenizing neutral density filtershown in FIG. 4( a);

FIG. 5 is a drawing for illustrating a light flux region at the surfaceA in FIG. 1 that later illuminates a particular point on the illuminatedsurface;

FIG. 6 shows a concentric structure of the plurality of refractivestructures according to one example of the illumination homogenizingoptical element of the present invention;

FIG. 7( a) shows an example of a cross-section of the illuminationhomogenizing optical element according to the present invention when therefractive structures are grooves in the surface of the optical element;

FIG. 7( b) shows an example of a cross-section of the illuminationhomogenizing optical element according to the present invention when therefractive structures are protrusions on the surface of the opticalelement;

FIG. 8 illustrates the surface angle θ and the deflection angle Φ (bothas measured from the direction of the incident rays) in the case wherethe refractive structures are grooves;

FIG. 9 is a graph of the deflection angle Φ (ordinate) versus thesurface angle θ (abscissa) of the refractive structures (i.e.,prism-shaped grooves or protrusions) of the present invention; and

FIG. 10 shows the ‘transmittance’ (as will be defined herein later)versus the distance from the optical axis of an illuminationhomogenizing optical element according to the present invention.

DETAILED DESCRIPTION

The present invention provides an illumination homogenizing opticalelement characterized by having refractive structures on at least oneoptical surface, such as prism-like grooves or protrusions, that arewider than the wavelength of light to be homogenized that is incident onthe illumination homogenizing optical element and these are positionedwith an interval between neighboring refractive structures that is widerthan the wavelength of light to be homogenized that is incident on theillumination homogenizing optical element. The refractive structuresremove a part or all of the incident light from the light path byrefracting a portion or all of the incident light out of the light pathdefined by the direction of the incident rays. The term “pitch” hereinrefers to a distance from a point on one diffractive structure to acorresponding point on an adjacent diffractive structure. When the widthand interval between adjacent refractive structures of the illuminationhomogenizing optical element are each greater than the wavelength oflight used, the main diffraction lobe of light that is diffracted by therefractive structures will subtend an angle of about 0.6 radians, whichallows such light to be entirely removed from the optical path (definedby the direction of the center ray of light incident onto theillumination homogenizing optical element) by the prism effect of therefractive structures. In this manner, the intensity of diffracted lightthat is not removed from the light path by refraction is made to besufficiently small that diffraction of light caused by the refractivestructures can be ignored.

The refractive structures may be formed as prism-shaped grooves in thesurface or as prism-shaped protrusions on the surface. The refractivestructures may be formed by molding processing or by cutting processing.In the case where the refractive structures are grooves, production bycutting processing is preferred. In the case where the refractivestructures are protrusions, production by molding processing ispreferred. In either case, the efficacy of the present invention doesnot essentially change. It is also preferred that the prism-shapedgrooves or protrusions each have one surface that is aligned with thelight incident onto the illumination homogenizing optical element. Inother words, one surface of the prism-shaped groove or protrusion hasits surface normal perpendicular to the direction of light that isincident onto the illumination homogenizing optical element.Furthermore, it is preferred that the refractive structures beconcentric and centered on the optical axis of the illuminationhomogenizing optical element.

Because a typical illumination non-uniformity has stronger illuminationnear the center than at the periphery of the illumination, it ispreferred that the pitch (defined above as being measured in linearunits) increases (i.e., the number of diffractive structures per mmdecreases) when proceeding from the optical axis center to the peripheryof the illumination homogenizing optical element. The illuminationhomogenizing optical element may be made of plastic, glass or any otheroptically transparent material.

It is also preferred that the illumination homogenizing optical elementof the present invention be utilized in an illumination device and, morepreferably, in a Keller illumination device.

The illumination homogenizing optical element of the present inventionmay be used as a component in a microscope, in a projector, or in astepper (e.g., a projection-type exposure device).

According to the present invention, an illumination homogenizing opticalelement with a low wavelength dependency is provided by controlling thetransmitted light according to the principles of geometrical optics,namely, by refracting out undesired portions of the incident light.Also, because the illumination homogenizing optical element of thepresent invention is realized using refractive structures such asprism-shaped grooves or protrusions, processing is very easy, andproducts with few individual differences can be manufactured.

Furthermore, because of a synergism between the calculations beingsimple (due to their being based on simple principles of geometricaloptics) and due to the optical processing of the illuminationhomogenizing optical element itself also being easy to manufacture,manufactured products that faithfully correlate with theoretical valuesin the design stage may be easily manufactured.

An embodiment of the present invention will now be explained withreference to the drawings. While the explanation given below is forhomogenizing the illumination for a microscope, the present invention isnot limited to such applications. For example, the present inventionalso may be applied to homogenize the illumination of projection devicesand exposure devices such as projectors and steppers. The presentinvention is thus applicable to any type of optical system wherein anillumination non-uniformity of a light source is to be corrected.

FIG. 1 shows a conventional illumination device that uses what is termedKeller illumination. A lamp housing 1 encloses a light source 2 and acollector lens 3 that converts diverging rays emitted from the lightsource 2 into substantially collimated light. A field stop 4 that limitsthe angular range of illumination is placed at a conjugate position toan illuminated surface 5. Substantially collimated light travels fromthe lamp housing 1, passes through the field stop 4, is reflected by amirror 9 which folds the optical path 90 degrees, and is then condensedonto a plane at an aperture stop 7 by a field lens 6. Then, the lightpasses through a condenser lens 8, and illuminates the surface 5.

If we assume that the Keller illumination shown in FIG. 1 can make theangular distribution of light emitted from the light source 2 to beuniform, illumination without any illumination non-uniformity can, inprinciple, be provide by the illumination optical system of FIG. 1.However, the angular distribution of light emitted by an actual lightsource is not uniform, causing illumination non-uniformity. A typicalnon-uniformity in illumination shows a higher illumination intensity inthe vicinity of the optical axis that decreases toward the periphery.

FIG. 2 discloses an illumination homogenizing optical element 10 that isinserted between the collector lens 3 and the field stop 4 forcorrecting an illumination distribution. Such a modification to theapparatus of FIG. 1, is taught in, for example, Japanese PatentApplication 2005-215992. In one embodiment of the illuminationhomogenizing optical element used in Japanese Patent Application2005-215992, the transmittance increases toward the periphery as shownin FIGS. 3( a) and 3(b). In another embodiment of the illuminationhomogenizing optical element used in Japanese Patent Application2005-215992, the transmittance increases toward the periphery as shownin FIGS. 4( a) and 4(b). Although both embodiments provide sufficientoptical performance to correct illumination non-uniformity, bothembodiments have some unsatisfactory aspects from the viewpoint ofcommercializing the invention.

In the present invention, an illumination homogenizing optical elementis provided that can effectively correct illumination non-uniformity,but the illumination homogenizing optical element of the presentinvention has a different transmittance distribution from those shown inFIGS. 3( a)-4(b). The transmittance distribution of the illuminationhomogenizing optical element of the present invention has atransmittance profile that is shown in FIG. 10, namely, the illuminationhomogenizing optical element of the present invention has steps ofconstant transmittance (that increase in width toward the periphery ofthe illumination homogenizing optical element) separated by regions ofzero ‘transmittance’ (as will be defined herein later). As a result ofthe increasing width of the constant transmittance regions toward theperiphery, the illumination homogenizing optical element of the presentinvention allows more light to travel straight through near theperiphery than near the optical axis, thus providing sufficient opticalperformance to illuminate a surface with even illumination while beingeasy and inexpensive to manufacture.

As is apparent by viewing FIG. 1, the light that is incident onto aparticular point of the illuminated surface 5 passes through a regionhaving a particular size in the illumination system. Referring to FIG.5, for example, light shining on a particular point that is a littledeviated from the optical axis at the illuminated surface 5 passesthrough a region Y at the plane A in FIG. 1. Note that, in FIG. 5, theregion X indicates the entire light flux at the plane A thatsubsequently is incident onto the illuminated surface 5. Thus, theillumination of the one point Y on the illuminated surface 5 can becontrolled by controlling the total amount of light passing through theregion X.

As illustrated in FIG. 6, in the illumination homogenizing opticalelement of the present invention, prism-shaped refractive structuressuch as the grooves 12 or protrusions 13 are formed concentrically on atleast one surface of the illumination homogenizing optical element, sothat the distribution of the prism-shaped refractive structure variesaccording to the distance from the optical axis. In addition, as shownin FIGS. 7( a) and 7(b), the cross section of the prism-shapedrefractive structures has one surface that is aligned with the lightincident onto the illumination homogenizing optical element. In otherwords, a surface normal of said one surface is perpendicular to thedirection of travel of the incident light.

The illumination homogenizing optical element in this embodiment canalso utilize grooves that are not concentric. For example, if theillumination non-uniformity is not rotationally symmetric about theoptical axis, the arrangement of the grooves should preferably not berotationally symmetrical about the optical axis. As will be explained indetail later, the present invention can provide sufficiently uniformillumination by utilizing refractive structures, such as grooves orprotrusions, which are arranged in a manner other than in a concentricpattern.

The illumination homogenizing optical element of the present inventionmay be a lens or a filter having prism-shaped refractive structures.Namely, the illumination homogenizing optical element of the presentinvention may be inserted in an illumination optical system as a newillumination homogenizing optical element, or an already-installedillumination homogenizing optical element may be processed so as to haveprism-shaped refractive structures. Needless to say, in a detachableinstallation form, it is preferred that the illumination homogenizingoptical element of the present invention be prepared as a newillumination homogenizing optical element.

While the installation position of the illumination homogenizing opticalelement relating to the present invention within an optical system is animportant design item, there are appropriate conditions to be satisfied.The installation position in a Keller illumination system as shown inFIG. 1 will now be discussed.

In a Keller illumination system that includes a condenser lens, it ispreferred that the illumination homogenizing optical element accordingto the present invention be placed in a conjugate position to a positionthat satisfies the following condition:

0.03<|L/f _(CD)|<0.4   Condition (1)

where

f_(CD) is the focal length of the condenser lens 8, and

L is the distance between the condenser lens 8 and the illuminatedsurface 5.

If the lower limit of the above condition is not satisfied, the positionwhere light passing through the illumination homogenizing opticalelement is projected toward the illuminated surface will be too near theilluminated surface, and grooves on the illumination homogenizingoptical element will be imaged at the image plane. On the other hand, ifthe upper limit is not satisfied, the position where light passingthrough the illumination homogenizing optical element is projectedtoward the illuminated surface will be too far from the illuminatedsurface 5, and it becomes difficult to obtain the effect of homogenizingthe illumination by eliminating the illumination non-uniformity even ifthe illumination homogenizing optical element of the present inventionis inserted. In addition, placing the illumination homogenizing opticalelement at a conjugate position that is closer to the light source ispreferable to placing it between the condenser lens 8 and theilluminated surface 5.

Referring to FIGS. 7( a) and 7(b), the width d of the groove 12 or theprotrusion 13 must larger than the wavelength λ of light utilized in theillumination system. Also, the interval D−d between neighboringrefractive structures must be larger than the wavelength λ of lightutilized in the illumination optical system. The reason for this isthat, otherwise, too much light diffracted by the refractive structureswill reach the illuminated surface, thereby preventing the objectives ofthe present invention from being achieved. Namely, the followingconditions must be satisfied:

d>λ  Condition (2A)

D−d>λ  Condition (2B)

where

d is a width of an individual refractive structure among the pluralityof refractive structures,

D is a pitch, measured in linear units, of the plurality of refractivestructures, and

λ is the wavelength of incident light.

For example, if the illumination homogenizing optical element of thepresent invention is used in the illumination device of an opticalmicroscope, it is preferred that the width d of a large majority ofrefractive structures be 1000 nm or larger. However, if the width d istoo large as compared with the diameter r of the illuminationhomogenizing optical element, shadows will be cast on the illuminationsurface. Therefore, it is preferred that the following condition besatisfied:

d/r≦0.001   Condition (3)

where

r is the diameter of the illumination homogenizing optical element, and

d is as defined previously.

Next, the geometrical optical actions of the refractive structures ofthe illumination homogenizing optical element relating to an embodimentof the present invention wherein the refractive structures areprism-shaped grooves will now be explained.

As shown in FIG. 8, a light beam a input onto a surface of theillumination homogenizing optical element such that the surface normalthereof is aligned with the incident beam goes straight through theillumination homogenizing optical element without being refracted. Onthe other hand, a light beam b incident onto a refractive portion (i.e.,in this case, a groove) is refracted an amount Φ (herein termed thedeflection angle) according to geometrical optics. In this process, ifthe deflection angle Φ is set to be sufficiently large, light beamspassing through the refractive portion are blocked by the outer diameterof a stop or another illumination homogenizing optical element. Althoughthe deflection angle value that blocks light also depends on theinstallation position of the illumination homogenizing optical element,a deflection angle of approximately 10 degrees or greater will usuallycause the deflected light to be blocked; thus this range of deflectionis desirable.

FIG. 9 is a graph of the deflection angle Φ (ordinate) versus thesurface angle θ (abscissa) of the refractive structures. It is desirablethat the surface angle θ be less than about 70 degrees. According to theusual definition of ‘transmittance’, light is transmitted even if it isrefracted. However, because light incident onto a refractive structuresuch as a groove or a protrusion portion is not utilized, herein theterm ‘transmittance’ will refer to the amount of light transmitted bythe illumination homogenizing optical element according the presentinvention and not eliminated by being refracted. Thus, herein, the‘transmittance’ at a groove or a protrusion portion of the illuminationhomogenizing optical element according the present invention will beregarded as being zero. Light can pass through the planar portions ofthe illumination homogenizing optical element according the presentinvention without being refracted. However, as is known, a certainamount of light will be reflected by a planar boundary merely due to thechange in the refractive index of the two media that form the boundary.For light that is normally incident at a boundary between twotransparent media, the transmittance T is given by:

T=1−[(N′−N)²/(N′+N)²]  Equation (A)

where

N and N′ are the indexes of refraction of the two transparent media thatform the boundary.

In addition, if the number of refractive structures per mm decreasesaccording to the distance from the optical axis (as illustrated in FIG.6), the portions having zero ‘transmittance’ also become more scarce asthe distance from the optical axis increases.

FIG. 10 shows the ‘transmittance’ (abscissa) versus the distance fromthe optical axis of the illumination homogenizing optical elementaccording to the present invention. The illumination homogenizingoptical element of the present invention is advantageous with regard tomanufacturing processing, as compared to the illumination homogenizingoptical elements shown in FIGS. 3( a) and 4(a). The illuminationhomogenizing optical element according to the present invention cancause the illumination distribution at a surface to be sufficientlyuniform despite the fact that the illumination homogenizing opticalelement of the present invention does not have a transmittance profile(such as the one shown in FIGS. 3( b)) that resembles the inverse of anillumination profile produced in an apparatus that illuminates a surfacewithout using an illumination homogenizer. In other words, theillumination homogenizing optical element of the present inventionfunctions adequately to make the illumination sufficiently ‘even’ oruniform at a surface in spite of the fact that it does not havetransmittance which precisely complements the illumination distributionof the illuminated surface when not using the illumination homogenizer.The reason is as follows. In the illumination optical system representedby FIG. 1, the relationship between the illuminated surface and thelight source is that of an image and a pupil, respectively. This is dueto the illumination non-uniformity on the illuminated surface beingcaused by the light intensity of light rays from the light sourcevarying with emission angle (as measured relative to the optical axis).On the other hand, the position (e.g., the plane A in FIG. 1) where theillumination homogenizing optical element of the present inventionshould be installed is neither an image position nor a pupil position,but is an intermediate position. The significance to the illuminationhomogenizing optical element is that the relationship between positionand aperture angle also has an intermediate nature. Thus, the positionof the illumination homogenizing optical element surface does not simplycorrespond to the illuminated surface position, as the relationship withthe aperture angle must also be considered. Because this property isutilized in the present invention, the illumination distribution of theilluminated surface can be made sufficiently ‘even’ (i.e., uniform) inspite of the fact that the illumination homogenizing optical elementdoes not have a transmittance that precisely complements theillumination distribution of the illuminated surface. In this manner,the present invention is similar to that of the prior art illuminationhomogenizing optical element shown in FIG. 4( a), but is easier andcheaper to manufacture.

Thus, the illumination homogenizing optical element of the presentinvention corrects a non-uniformity in illumination of a surface whilehaving a transmittance distribution that does not directly cancel theintensity distribution of the light source.

The invention being thus described, it will be obvious that the same maybe varied in many ways. For example, there may be a limited number ofrefractive structures having a width that is not wider than thewavelength of light incident onto the illumination homogenizing opticalelement, or a limited number of adjacent refractive structures may nothave an interval longer than the wavelength of light used, so long asthe ratio of: the surface area of such more narrow refractive structuresand intervals is sufficiently small that diffraction by said more narrowrefractive structure and intervals can be ignored. Also, the opticalrefractive structures of the illumination homogenizing optical elementcan be formed on one surface of a conventional optical element such as afilter or a lens. Such variations are not to be regarded as a departurefrom the spirit and scope of the invention. Rather, the scope of theinvention shall be defined as set forth in the following claims andtheir legal equivalents. All such modifications as would be obvious toone skilled in the art are intended to be included within the scope ofthe following claims.

1-21. (canceled)
 22. An illumination device that includes a condenserlens, that has an image position and a pupil position, and that includesan illumination homogenizing optical element having a plurality ofrefractive structures arranged on at least one optical surface, anindividual refractive structure among the plurality of refractivestructures being wider than the wavelength of light to be homogenizedand used to illuminate a surface, said illumination homogenizing opticalelement having a wider interval between neighboring refractivestructures than said wavelength of light and being positioned other thanat an image position or a pupil position of the illumination device. 23.The illumination device according to claim 22, wherein the illuminationhomogenizing optical element is installed at a position that is anoptical conjugate to a position which satisfies the followingconditions:0.03<|L/f _(CD)<0.4 where f_(CD) is the focal length of the condenserlens, and L is the distance between the condenser lens and theilluminated surface.
 24. The illumination device according to claim 22,wherein the illumination homogenizing optical element is formed on asurface of a filter.
 25. The illumination device according to claim 22,wherein the illumination homogenizing optical element is used in amicroscope.
 26. The illumination device according to claim 22, whereinthe illumination homogenizing optical element is used in a projector.27. The illumination device according to claim 22, wherein theillumination homogenizing optical element is used in a stepper lightexposure device.